1. Field of the Invention
This invention relates to molded plastic fluid conduits and more particularly to thin-walled, rigid, transparent fluid conduits suitable for use in surgical irrigation-aspiration apparatus.
2. Description of the Prior Art
In performing delicate surgical procedures within the anterior chamber of the eye, it is necessary to make at least one penetrating incision passing through the peripheral tissues of the eye such as the cornea, the sclera, and the like. A particularly common procedure is the removal of a cataract by ultrasonic fragmentation and aspiration through a small-diameter ultrasonically vibrated needle having an axial aspiration conduit, which is inserted through an incision just large enough to accomodate the diameter of the tool. Fluid leakage though the incision as well as removal of aqueous humor by aspiration through the ultrasonic needle leads to loss of the aqueous humor from the interior of the eye. If this fluid is not replaced, the anterior chamber of the eye will collapse, with serious consequences for the health of the eye, particularly for the delicate corneal endothelium. Accordingly, in order to prevent this collapse it is conventional to provide a supply of irrigating fluid to the interior of the eye, either through a secondary incision in the eye or by means adapted for both irrigation and aspiration through a single incision. The single incision method is generally preferred, but it requires an irrigation-aspiration apparatus having a double lumen tube with the separate channels for irrigation and aspiration placed either side by side or coaxially. The coaxial configuration has certain advantages, especially for ultrasonic surgical instruments having provision for irrigations and aspiration. Such an instrument which incorporates an ultrasonic fragmenting and aspirating needle and a coaxial irrigation conduit is disclosed, e.g., in Banko and Kelman, U.S. Pat. No. 3,589,363.
The performance requirements of a sleeve which defines the irrigating channel in a coaxial surgical irrigator/aspirator place severe demands on the materials and design of the sleeve. The sleeve should have a low heat conductivity to insulate the tissue at the sides of the incision from heat generated by friction with the ultrasonic tool. It should be sufficiently strong to prevent collapse under pressure from the sides of the incision. At the same time it should be as of a small diameter as possible to allow a minimum size incision. It is also helpful if the sleeve is transparent to provide better visibility for the surgeon. Finally, because the element is small and of rather precise dimensions, it is desirable that it be capable of being formed by molding for economical manufacture.
The materials hitherto used in the fabrication of such irrigation sleeves have each had certain drawbacks. Metal sleeves, e.g., stainless steel, have the advantage of rigidity, which makes them easier to insert through a small incision and less likely to injure the ocular tissues such as the corneal endothelial cell layer and Descemet's membrane. Since they cannot be collapsed by pressure from the sides of the incision, they make it easier to assure a constant flow rate of irrigation fluid. However, because of their extreme rigidity, they must be very precisely matched to the shape of the interior aspiration conduit, so that many different sleeves are needed to accomodate various aspiration conduits. Furthermore, because of their heat conductivity, they do not protect the adjacent tissues from heat generated when the aspiration conduit is an ultrasonically vibrated surgical tool. Accordingly, metal sleeves have hitherto not been used with ultrasonic surgical instruments but only with simple irrigation-aspiration devices.
Synthetic resins, in the cross-sections required for irrigation sleeves, tend to be lacking in rigidity and/or strength. This requires a larger incision, either to accommodate a larger wall thickness or to minimize the force from the sides of the incision which tends to collapse the sleeve. Furthermore, some plastics are not transparent, and, like metal sleeves, obscure the surgeon's vision. U.S. Pat. No. 3,589,363 recommends the use of a sleeve of polytetrafluoroethylene, which is inert, but difficult to fabricate and is not perfectly transparent, even in thin sections. A commonly used commercial irrigation sleeve is made from silicone rubber and is molded integrally with a cap for fastening the sleeve to the ultrasonic handpiece. Silicone rubber can be conveniently molded by injection molding, but is flexible and tends to collapse under pressure from the incision and to fold back on itself or "telescope" when it is inserted through the incision. Furthermore, it is at best translucent, so that the surgeon's vision is somewhat obscured.
Hence a need has continued to exist for a surgical irrigation conduit which is free from the drawbacks of the known irrigation conduits.